High-resolution radio observations of the colliding-wind binary WR 140

Milliarcsecond resolution Very Long Baseline Array (VLBA) observations of the archetype W-R+O star colliding-wind binary (CWB) system WR 140 are presented for 23 epochs between orbital phases 0.74 and 0.97. At 8.4 GHz, the emission in the wind-collision region (WCR) is clearly resolved as a bow-shaped arc that rotates as the orbit progresses. We interpret this rotation as due to the O star moving from southeast to approximately east of the W-R star, which leads to solutions for the orbital inclination of 122 degrees +/- 5 degrees, longitude of the ascending node of 353 degrees +/- 3 degrees, and an orbit semimajor axis of 9.0 +/- 0.5 mas. The distance to WR 140 is determined to be 1.85 +/- 0.16 kpc, which requires the O star to be a supergiant. The inclination implies that the mass of the W-R and O star is 20 +/- 4 and 54 +/- 10 M-., respectively. We determine a wind momentum ratio of 0.22, with an expected half-opening angle for the WCR of 63 degrees, consistent with 65 degrees +/- 10 degrees derived from the VLBA observations. Total flux measurements from Very Large Array (VLA) observations show that the radio emission from WR 140 is very closely the same from one orbit to the next, pointing strongly toward emission, absorption, and cooling mechanism(s) that are controlled largely by the orbital motion. The synchrotron spectra evolve dramatically through the orbital phases observed, exhibiting both optically thin and optically thick emission. We discuss a number of absorption and cooling mechanisms that may determine the evolution of the synchrotron spectrum with orbital phase.